Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, natural gas has been recognized as an attractive alternative fuel. For automotive applications, natural gas may be compressed and stored as a gas in cylinders at high pressure. A pressure regulator may then be used to supply the compressed natural gas (CNG) at lower pressures to an engine combustion chamber. The pressure regulator may provide this gaseous fuel at a fixed, constant pressure to the engine, or it may be a variable pressure regulator which can provide gaseous fuel at varying pressures to the engine.
However, one unresolved issue with pressure regulators for gaseous fuel is their tendency to overshoot pressure when the engine is turned on. For example, during cold start, a pressure regulator may not be warmed up, and thus may not be capable of regulating gaseous fuel pressure to a desired pressure (e.g., lowering the pressure of high pressure gaseous fuel from a high pressure fuel tank). Further, during engine cold start, even a variable pressure regulator may have residual high fuel pressure in the fuel rail. A relatively high voltage may be needed to open the injectors against the high fuel rail pressure during cold start conditions. However, extreme cold starts may have only lower voltages available for injector operation. Thus, whereas fuel injector opening may be used to depressurize the regulator during other engine operating conditions, this means may be unavailable during cold start conditions. Without means for depressurization during cold start conditions, the pressure in the fuel rail during cold start conditions may be undesirably high, as the fuel rail may be stuck at a high pressure until high enough voltages are available to open the injectors to drain off some of the pressure. The undesirably high fuel rail pressure during cold start may negatively affect fuel economy and engine performance. In addition to cold start conditions, overpressure may also be an issue during other engine operating conditions where fuel injectors are not active and thus cannot be used to drain excess fuel pressure from the pressure regulator.
To address the above issues, the inventors herein have recognized that pressure overshoot during conditions where fuel injectors are inactive (e.g., cold starting) may be reduced by means other than fuel injector opening. For example, pressure overshoot may be reduced by opening a valve communicating a low pressure chamber of a mechanical pressure regulator with the reference chamber of the regulator, while simultaneously opening a valve exhausting the reference chamber, for example a fuel vapor storage canister, an engine crankcase, or an engine intake manifold. In this way, even when the regulator has not yet begun regulating gaseous fuel pressure in accordance with its normal principle of operation and the injectors are not available to drain excess pressure from the fuel rail, the valves may be controlled to depressurize the reference chamber of the pressure regulator, thereby avoiding excessive pressure at the fuel rail. Further, in examples where the reference chamber exhausts the high pressure gaseous fuel to a fuel system component such as the fuel vapor canister or to an engine component such as the intake manifold or the crankcase, the depressurization method may be performed without negatively affecting fuel economy as the exhausted gaseous fuel may ultimately be routed to the engine for combustion. While the depressurization method is performed, the engine may or may not be running on a second fuel (e.g., a liquid fuel).
The inventors herein have also recognized additional advantages that may be achieved by incorporating a valve communicating the low pressure chamber of a mechanical pressure regulator with the reference chamber of the regulator and a valve exhausting the reference chamber, for example to another component or system of the vehicle. The valve communicating the low pressure chamber of the regulator with the reference may be controlled to flow gaseous fuel from the high pressure chamber to the reference chamber to increase the pressure of the reference chamber, and the valve controlling exhausting of the reference chamber (e.g., to the fuel vapor storage canister, intake manifold, crankcase, ejector vacuum, or vacuum pump vacuum) may reduce the pressure of the reference chamber. In this way, the regulator may regulate gaseous fuel pressure to different pressures via control of the valves, which may effectively transform the mechanical pressure regulator from a fixed-pressure regulator to a variable pressure regulator. Many advantages may be achieved by using a variable pressure regulator to provide gaseous fuel to the engine, instead of a pressure regulator which provides gaseous fuel to the engine at a fixed, constant pressure. For example, varying the pressure of gaseous fuel increases the dynamic range of the injector and allows rare, peak fuel demands to be satisfied without having to subject the injector to the durability challenge of injecting high pressure gaseous fuel at all times. Whereas known variable pressure regulators may be costly, prone to instability, and subject to pressure overshoot during cold start conditions, the pressure regulation system described herein may enable fuel depressurization while fuel injectors are inactive (e.g., during cold start), and provision of fuel at varying pressures to the fuel rail via control of the two valves while fuel injectors are active (e.g., after cold start). Advantageously, the valves may be small and inexpensive, and yet the system may still outperform variable pressure regulation approaches which involve duty cycling a main valve between the regulator and the fuel rail.
In one example, a method for regulating gaseous fuel pressure in an engine comprises, when gaseous fuel injection is inactive, flowing gaseous fuel from a low pressure chamber of a pressure regulator into a reference chamber of the regulator and exhausting gaseous fuel from the reference chamber, e.g. to a fuel vapor storage canister. Even during conditions where the fuel injectors are inactive and cannot be used to depressurize the fuel rail, this method may reduce fuel rail pressure overshoot without negatively affecting fuel economy, as the fuel exhausted from the reference chamber is directed to a component such as the fuel vapor storage canister (and ultimately to the engine for combustion). Importantly, fuel economy may be preserved via this method even without a fuel return line (e.g., a line returning fuel to the high pressure fuel tank), which may not be a practical option when using high pressure gas such as CNG as the depressurized gas would require compression before being returned to the high pressure fuel tank.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.